Accurate measurements of the rate at which cells turn over in tissues are of critical importance in understanding disease processes For example, among the more important questions regarding the pathogenesis of AIDS is how fast does CD4+ T cell destruction and replacement occur in the various stages of HIV infection? Therefore, we set out to develop a method of measuring cell cycle kinetics in rhesus macaques by utilizing a single intraperitoneal dose of BrdUrd administered hours before death in animals Although this method is routinely used in rodents, no non-human primate studies have been performed using this method of BrdUrd administration To determine the optimal time for cell labelling, animals were intraperitoneally inoculated with BrdUrd (80mg/kg) at 2, 4, 24, 48, hrs and 5 days prior to sacrifice Immunohistochemistry and morphometric analysis of intestinal epithelial cell BrdUrd reactivity was determined Excellent labelling of dividing (S-phase) crypt epithelial cells was observed as compared to PCNA or MIB-1 which both labelled essentially all crypt cells with significantly more background staining BrdUrd labelled cells in early time points (2 and 4 hrs) were limited to the basolateral crypt cells, which is consistent with known intestinal physiology By 48 hrs, most of the in testinal labelling had disappeared or was limited to the villus tips Quantitative image analysis demonstrated that a 2 to 4 hour interval was the optimum time for labelling cells in the S phase We are currently comparing intestinal epithelial and lymphocyte proliferation between uninfected and SIV infected animals to determine the effects of SIV infection on cell proliferation in various lymphoid tissues